The growth in container shipping over the past decade has created some special problems and opportunities for the commercial trucking industry. In particular, shipping containers are remarkably strong and even a twenty-foot container is capable of carrying a concentrated load that is in excess of that normally carried in a 40 foot van. As distribution centers for import and export of bulk and consumer goods have become less centralized in recent years, the challenge has been to find ways of transporting heavy containers on state and federal highways. Standards established by Federal and state departments of transportation have established standard for roadways and bridges designed to spread the load.
The Federal Interstates Bridge Laws (FBL) sets forth a formula (W=500[LN/(N-1)+12N+36]) for calculating the maximum allowable weight carded by any group of 2 or more axles, wherein W is the maximum weight carded by any group of 2 or more axles, L is the distance in feet between the extremes of any group of 2 or more consecutive axles, and N is the number of axles. According to the formula a 20 foot trailer with 2 axles can have a maximum allowable weight of only 50,000 (including the weight of the trailer chassis). In the 1970-80's the only West Coast ports capable of off-loading containers onto the public roads were in the states of Oregon and Washington and in Vancouver, British Columbia. As an example of load limits, the Department of Transportation (DOT) for the State of Washington administers state law 46.44.041; (amended by Chapter 102, Laws of 1993) as follows: "No vehicle or combination of vehicles shall operate upon the public highways of this state with a gross load on any single axle in excess of twenty thousand pounds, or upon any group of axles in excess of that set forth in the following table, except that two consecutive sets of tandem axles may carry a gross load of thirty-four thousand pounds each, if the overall distance between the first and the last axles of such consecutive sets of tandem axles is thirty-six feet, or more." The data in the accompanying DOT table shows a maximum weight of 40,000 pounds for twin axle trailer and 60,000 for a trailer with three axles at a distance between the consecutive sets of axles of 32 feet. Shipping containers are commonly about twenty or forty feet in length and a 20 foot container may be capable of carrying 50,000 pounds. The rising popularity in use of maximum-weight containers (also abbreviated herein MWC), particularly for hauling bulk cargoes such as hay, apples, and mineral ores have created special challenges for truck trailer chassis designers. Trailers slidably extending in length are one solution that has been disclosed in the art (e.g., see U.S. Pat. Nos. 4,400,004; 4,580,805; and 4,969,659).
Use of trailers for hauling MWC has revealed limitations in the art. When a chassis is extended to allow a 20-foot MWC to be carried, the distance between the axles increases and the container load is centrally loaded on the chassis, i.e., a "bridging" load. The central loading puts heavy "flex stresses" on the trailer and it is not uncommon for weld fractures and stress cracks to develop in heavy steel I-beams. The art has attempted to solve this problem by using deeper I-beam construction with thicker 1/2-5/8 inch top and bottom flanges. However, this adds significantly to the total weight of the trailer which in turn adds to fuel consumption and operating cost while at the same time reducing the maximum allowable FBL load. The use of triple axle has revealed still other limitations in the art. When turning or maneuvering long triple axle trailers loaded with MWC the chassis are exposed to tremendous "side load stresses", and weld cracks and longitudinal I-beam stress fractures may occur, sometimes within even a few weeks of use. An example of a cross-section of such a conventional I-beam trailer is shown in FIG. 1, where arrows point to where weld and frame failures may take place.
Traditional welded I-beam truck trailer chassis construction strengthened by gusset plates and angle irons, suffers from a number of disadvantages: namely,
First, the I-beam structure is uniform in strength and weight, even at stations where less structure (and weight) are needed;
Second, to prevent fracturing in the middle of the bridging I-beams are commonly of a greater depth (and weight) in the entire beam than might be necessary. Greater beam depth, however, means added weight and decreased flexibility;
Third, trailers chassis having three or more axles, frequently exhibit weld fractures at the junction of the lateral and longitudinal I-beams, arrows FIG. 1. Longitudinal I-beam failures are also not uncommon. Adding extra bracing at the junction between the lateral and longitudinal I-beams, e.g., with angle irons, adds manufacturing cost and weight; and,
Fourth, the strategy of adding a third axle and lengthening the trailer creates problems for the operator because it is difficult to load and unload trailers if the container is centrally located on a long span.
Since hauling MWC is a relatively new commercial enterprise the lifespan of most trailers is not clear, but recent experience suggest that it may be as short as 1-3 years. The replacement cost of a container trailer chassis is currently in the range of $18,000 to $50,000, making longevity a very desirable attribute.
It is thus a particular object of the present invention to solve the problems in the art by providing a strong chassis capable of hauling heavy loads, withstanding heaving bridging loads without fracturing and while minimizing weight, and operating and manufacturing costs.